Bigger is always better.
What is the absolute largest tank you will consider?
Dont go smaller than 2.5g, but if you can fit larger and can afford it, you will also enjoy a bit larger. It is true--the smaller tanks are a lot more work to keep stable and require more diligence with keeping up on them.
What size you go for will decide what heater or filter you need.
In both my betta tanks, i run an Aqueon Mini Heater 10w. Its preset but it keeps their tanks a comfy betta temp at 79-80F.
Theyre small and easy to hide behind plants, rocks, etc. BUT always unplug these ones when doing water changes. They automatically shut off if exposed to air and wont turn back on.
As for filters, my 3.5g came with a filter. Its a very gentle filter, but i still shove filter sponge through the exit anyways. Both slows the water flow (i use floating plants) and adds more biological filtration. I chucked the little cartridges that came with the filter. Cut up filter sponge fit into slots works way better. I also use a bit of Fluval Clearmax in it too because i unfortunately have nitrates already in my tap water.
My 2.5g i made the filter using a small bottle... drilled a few holes in the bottom and a few in the top. Added 1 larger hole in the side near the top and inserted an airline. Inside the bottle is filter sponge and ceramic media. Its gentle and does the job.
Smaller tanks require more drastic water changes. I do 90% weekly on my betta tanks plus about 30% daily. Check your parameters frequently. Nitrate absorbing fast plants are heavily recommended. Hornwort, anacharis, and floating plants especially all are excellent. The first two can be floated or anchored/planted (hornwort needs to be anchored or floated, it wont do well planted. I anchor mine down with suction cups)
As for keeping a betta from being bored, changing up their tank often helps that with smaller tanks, as does adding leaf litter (indian almond leaves are your safest bet). This goes for any tank size for a betta though.
Filters...
This is the setup for my girl, its blackwater so its stained with tannins from the leaves and driftwood. The wood is still new so its going through that awful fuzzy stage lol
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The filter for it:
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And parameters for this tank: this is ideal for small set ups to aim for. No ammonia, no nitrites, and keep nitrates below 20, better even under 10.
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And my male set up, his is the tallest one. The two jars are snail jars. (I raise ramshorns)
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His filter on the inside, so you get an idea of the sponge i mentioned...
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A small set up isnt always ideal, but its not necessarily cruel or terrible. Bigger is always better. Depends on the work you are willing to put into the tanks. I could afford to miss a water change on my 46 gallon for example. My betta tanks would not be so forgiving at all!
